Related T335

Related T335
Related T933

Closes: T335

reincarnation of the old shw.model bin/ script, which is now gone

Summary:
Add support for snapshot identifiers

Close T566.
Related to D268.

Test Plan: Unit tests included

Reviewers: zack, #reviewers!

Maniphest Tasks: T566

Differential Revision: https://forge.softwareheritage.org/D277

Close T709

This allows loaders to lazily load data: you can read from disk and transfer
only the contents that are really missing.

Summary:
This module is a reimplementation of swh.model.git, with the underlying goal of
replacing it and fixing T709 in the process.

Reviewers: #reviewers!

Differential Revision: https://forge.softwareheritage.org/D248

Currently just a stub (but with a hereby fixed anchor) to allow linking to it
from other part of the development documentation. Will be fillen later on with
an abstract description of the Software Heritage data model.

change cherry picked from python module template
commit 71b117ba0cf9f1251b1cac26d0994df03a4c787d

Summary: for those people with legacy openssls

Reviewers: anlambert, #reviewers!

Differential Revision: https://forge.softwareheritage.org/D224

Fix T685 out of spite.

While we wait for a cleaner refactoring of this code, this fixes the immediate
clogging of the database with bogus data issue.